The present invention relates to insulation systems and more particularly concerns multilayer systems embodying prefabricated modules.
Cryogenic liquids, particularly liquified gases, have been stored and transported for many years. Such materials must be confined under great pressure if at ambient temperatures, or must be maintained at cryogenic temperatures if confined near atmospheric pressures. The cost and dangers inherent in high pressure containers of large volume provide the cryogenic container with significant advantages. Accordingly, thermal insulation systems have been utilized in containers for storage and transportation of liquified gases.
Multilayer cellular foam insulation, cork, pearlite, fiberglass, cellular rubber, cellular glass and many other materials, have been employed in cryogenic containers. Several criteria are of primary importance in the selection of insulation systems. A first one of such criteria is low thermal conductivity and a second is adequate mechanical strength. A third criterion is ease of fabrication and installation. The latter is economically significant because even a small decrease in unit cost of installation can result in substantial savings over the literally acres of insulation that may be required for a liquid natural gas tanker.
The cellular foams, and in particular, urethane foam, are among materials having lowest thermal conductivity. Thus, in the aforesaid patent of Isenberg and Hillberg, there is disclosed a system making use of the exceedingly favorable thermal characteristics of the urethane foam and, in addition, meeting requirements of various laws and regulations. Such laws and regulations are, in general, concerned with safety of large containers of the potentially dangerous liquified gases. Because of the high risks involved, both primary and secondary liquid barriers are required, and in addition, suitable means must be provided to detect and warn of damage or leak. Thus, the system of the Isenberg and Hillberg patent employs a liquid barrier membrane between plural layers of foam insulation and includes a system for monitoring leakage. The Isenberg and Hillberg system provides significant economies with respect to prior foam systems, since the latter generally required the mounting of the foam within rigid protective structural shells prior to installation. Such shells have been required for dimensional stability and to form both liquid and vapor tight barriers. This type of insulation system is described in the several patents assigned to Esso Research and Engineering Company, Nos. 3,341,051, 3,381,843, 3,341,050, 3,363,796 and 3,339,780.
Although polyurethane foams provide efficient insulation systems in temperature ranges between +200.degree.F. and -100.degree.F., they become brittle and exhibit poor mechanical properties at cryogenic temperatures in the range of -150.degree.F. to -452.degree.F. Further, they are subject to widespread surface cracking and structural failure. Thus, the copending application of Mack and Helf for Thermal Insulation With Relief of Thermal Stress, Ser. No. 286,512 discloses a modification of the system of Isenberg and Hillberg in which the inner layer of foam isulation is formed with stress attenuating discontinuities thereby avoiding surface cracking failures. In one arrangement of the discontinuities, the innermost layer of insulation, namely, that which is in contact with the confined liquified gas, is formed by a number of edge-to-edge blocks having barrier strips mounted at the joints between adjacent blocks to provide a primary liquid barrier.
The application of urethane foam whether poured or sprayed in place, or preformed in blocks that are bonded in place, requires careful and precise control of many conditions and parameters including careful environmental control. Various factors, such as humidity, temperature and air borne contaminants, must be carefully controlled in the application of urethane adhesives and in the formation of the urethane foam itself. Where the insulation is to be applied to and within the hold of a tanker forming several containers, each in the order of 100 .times. 120 feet by 80 feet high for example, significant problems may be encountered in the control of these environmental parameters. Accordingly, it is preferable to perform as much of the manufacture and assembly of the systems as possible in a shop where greater control and precision of operation is possible.
It is an object of the present invention to provide improved insulation systems and methods that decrease time and effort required for installation without degradation of operation of the system.